Where have the carriages gone

It's the same on the Woolwich branch, although the proper terminology is that the trains are made up of 2 or 3 cars^[1], carriages or units (all three terms seem to be used^[2]).

As to where they have gone, my guess is that an element of it is overdue maintenance. The fleet was stretched right to the limits during the Olympics, and that level of usage is almost certainly going to be unsustainable in the long term. However, before the Olympics almost all Bank/Tower Gateway services to the eastern branches were 3-car while 2-car trains predominated on the Stratford Internetional route. This past week or so I've been on a couple of Woolwich-Bank trains that were 2 car services and haven't seen a 3-car formation heading to Stratford International in quite a while. I don't travel on the Isle of Dogs routes regularly enough to know how long the trains there are at the moment.

[1] They can of course also be formed of one car, but the only time I've seen a single car outside a depot was a couple of years ago when a failed unit was being recovered to Beckton Depot at walking pace. It was accompanied by about half a dozen staff walking ahead/behind it and was preceded by a three-car train (almost certainly a 2-car rescue train and the other carriage from the failed train).

[2] Although on the first day of longer trains on the Beckton branch, the platform indicators at Cyrpus were advertising that "3 car carriages" were now being operated, which is wrong whichever way you look at it.

Bank-Lewisham is solid 3 car

I have just discovered that the DLR frequencies page on the TfL Website advertises which routes get 3-car services:
Bank-Lewisham (all day), Bank-Woolwich Arsenal (all day), Tower Gateway-Beckton (all day) and Stratford-Lewisham (06:30-10:00).

I'm not the OP, but from experience there have been a handful of occasions since about mid-October when I have travelled on or seen 2-car services on the Woolwich-Bank route off-peak. Although I haven't seen any 2-car Tower Gateway-Beckton trains, their existence on the other eastern branch gives plausibility for their existence.
Today at about 3:30-4pm-ish I travelled on a 3-car Stratford (presumably) to Canary Wharf service southbound from Poplar, despite this route being advertised as 2-car only after 10am.
I don't travel with sufficient regularity or frequency for my sightings to be more than anecdotal though.

Indeed, but I'm specifically not talking about the Stratford International routes.

This weekend, the Woolwich-Tower Gateway/Shadwell service was formed of two-car trains, while Canning Town-Stratford International was operated with three car trains which seems rather odd given the typical loadings. The Jubilee Line was fully working, so that obvious reason for more traffic on the SI branch doesn't apply.

Could well be.

Minor edit by Londonstuff: accidental double post removed.